The present invention relates to optics and, more particularly, to lasers. A major objective of the present invention is to provide low divergence output from a linear array of semiconductor lasers.
Lasers are a well-known source of coherent light. This coherency allows the laser light to be readily controlled, directed and concentrated. In addition, laser output tends to be limited to certain resonance modes; thus, laser light is often highly monochromatic. Accordingly, lasers serve in many applications as high-intensity precision light sources, as high intensity precision heat sources, and as signal carriers. Semiconductor lasers promise these capabilities in an inexpensive, compact, and rugged format. In fact, semiconductor lasers are the prime candidate for signal generators in the heavily promoted vision of an information superhighway.
Single-mode semiconductor lasers can provide coherent light. However, such lasers tend to provide only milliwatts of optical output. There are two basic approaches to increasing the optical power obtainable from semiconductor lasers: 1) develop more powerful semiconductor lasers, and 2) combine semiconductor elements in an array.
Taking advantage of both of these approaches, high-power "broad-area" semiconductor lasers have been arranged in linear arrays. The term "broad-area lasers" encompasses any laser that can support multiple spatial modes. Broad-area lasers can themselves be laser arrays, so herein the arrays they are incorporated into are referred to as "super arrays", and the output of a super array is herein referred to as a "super beam". The super beam is constituted by the beams from individual broad-area lasers. Each broad area laser can have a spatially periodically varying gain to exclude certain undesirable operating modes. The periodicity establishes discrete high-gain regions so that the broad area output beam is constituted by a linear series of subbeams.
The problem with these broad-area arrays and arryas is the lack of coherence among the beams and among the subbeams. This lack of coherence decreases the spectral purity, the controllability, and the general usefulness of the output for traditional laser applications. Several approaches to improving the coherency among the several subbeams of a broad area laser have met with at least limited success.
For example, light can be fed into the back face of a broad area laser by an external mirror, thus defining an external cavity. If the mirror is on-axis, then a spatial filter (an aperture) can be placed in the external cavity to prevent certain transverse modes from resonating, thus forcing the laser to operate in its fundamental, in-phase transverse mode. A disadvantage of this approach is that much of the available power is lost due to the filtering, and that there is no separate output beam. One can also feed light into the back face of a broad-area laser, but then it becomes more difficult to cool the laser because optical access is then needed to both the front and back laser faces.
Alternatively, a mirror can be arranged off-axis to feedback a portion of the output back into the front face of the laser. This arrangement can enhance the operation of a high order transverse mode. However, this arrangement is very sensitive to mirror misalignment, for example, due to mechanical vibrations and temperatures changes.
The alignment problem can be addressed by using a phase-conjugator instead of a mirror. While a mirror only returns a normal incident beam back to its source, a phase conjugator returns incident light arriving from any of a wide range of angles back to its source. A phase conjugator can adjust to small changes in alignment in real time so that laser operation is not disturbed by vibrations and temperature changes. However, the lack of coherency from a broad area laser makes it difficult to initiate phase conjugation. S. MacCormack and R. W. Eason, Opt. Lett. 16, 705 (1991) addresses this problem using a separate single-mode laser to drive the broad area laser. The broad area laser's subbeams are all locked to the single-mode laser, providing the coherency necessary to initiate phase conjugation. There is an alignment issue in directing the single-mode laser's output into the broad area laser, but this is much less critical that the alignment of a feedback mirror. However, the requirement of the additional laser adds to the cost and complexity of the laser system.
It should be noted that none of these methods works very well when extended to the super array of broad area lasers. In part, this is because any required laser and/or mirror alignment must be relative to many lasers at once. What is needed is a laser system that can take advantage of the power available from a super array of broad beam lasers and yet emit a laser beam that has the qualities of extreme directionality and coherence that make lasers so useful. Moreover, it is desirable that such a system does not require an external laser to drive the main output lasers.